Why do I get an error telling me I don’t have any available web services sessions?

The most likely cause is that you have Web Services R16 Compatibility Mode turned on. This mode is used for maintaining older systems of Oracle CRM On Demand and may no longer be necessary. Otherwise, your CRM system might be under heavy load right now and you may need to wait a few minutes before trying again. If you get this error repeatedly, it may be worth contacting Oracle to get your overall concurrent session limit raised.